1. Field of the Invention
The present invention generally relates to an image forming apparatus, more particular, to the image forming apparatus provided with an image forming unit for forming a single color toner image onto an image transfer medium, such as a paper, wherein a toner image of full colors is formed by accumulating the single color toner image.
2. Description of the Related Art
There are many methods for forming a color image on an image transfer medium, such as a paper. For example, an image forming apparatus for the color image based on electrophotography comprises a photoconductor as an image forming medium and an intermediate transfer body, namely a single photoconductor drum type apparatus. As another example, there is known a tandem type apparatus in which a plurality of image forming unit are arranged in succession.
The single photoconductor drum type apparatus comprises four color developing bodies for three colors (yellow, magenta and cyan) and black for the purpose of formation of color images on a periphery of the single photoconductor drum. This apparatus employs a technology in which one color toner image is formed on the photoconductor drum by means of each developing body and is then transferred onto the intermediate transfer body sequentially, and the foregoing steps are repeated to form full color toner images thereon which are in turn transferred onto a recording material. On the other hand, the tandem type apparatus comprises four image forming units for three colors and black in order for each unit to form the single toner image. This apparatus is based on a technology that a single color toner image is successively accumulated on a paper or the like which is arranged in a transporting direction, thereby producing the color images.
In any types of the color image forming apparatus described above, the single color toner image is successively transferred and then accumulated so as to form the color image. When toner image is transferred, a subsequent toner image is accumulated on the previous toner image except that a yellow toner image is transferred. Thus, because of repetition of such a transfer step, a toner image which was previously formed on the paper or the like may adhere to other photoconductor drum (which is a photoconductor drum on a downstream side) in a subsequent transfer step. That is to say, there may be arisen problems that a reverse transfer phenomenon takes place.
Under these circumstances, it is proposed an image forming apparatus in which an improvement is made for a decrease in a toner deposition amount on the paper due to the reverse transfer phenomenon. For example, it is proposed a control method of the toner deposition amount on the paper, wherein a developing roll in a developing unit has a surface roughness different from that of other developing roll, as disclosed in Japanese Laid-Open Patent Application No. 11-102112. However, in this case, there may exist a need for preparation of the developing rolls dedicated to each of the developing unit, so that manufacturing cost is increased and it is required to have much time and costs for apparatus maintenance.
Japanese Laid-Open Patent Application No. 6-83151 describes a control method of the toner deposition amount on the paper, by which a latent image strength on the photoconductor drum or development bias of the developing unit is controlled. However, in this case, it is necessary to establish bias conditions for each color. This cases the apparatus to be manufactured more expensively due to complexity of control content.
Japanese Laid-Open Patent Application No. 9-319179 describes a control method of the toner deposition amount on the paper, wherein a toner particle size is controlled. The method is based on the fact that the toner deposition amount is increased with increase in a particle size, so that the particle size can varies with colors. However, in this case, there may be possibility of image quality difference according to colors. The toner particle size constitutes a causative factor in image quality. If the toner particle size is large, image quality is likely to degrade. In a case of the color toner where the deposition amount is accurately controlled by increasing the toner particle size, since uniformity of a deposition amount onto the paper as a whole is likely to be irregular, it is difficult to produce images with acceptable quality.
In addition, Japanese Laid-Open Patent Application No. 9-319179 also describes a control method of the toner deposition amount on the paper, wherein toner fluidity is controlled. However, in this case, problems are encountered in image quality on account of difference of the toner fluidity between the colors. When the toner fluidity is poor, there may be possibility of solidification of the toners due to a strong aggregation force between the toners. This results in degradation of image quality with the contour of the image being deformed, as observed in the case of using the large toner particle size, thereby providing problems relating to defective appearance of the image.
As described above, although there has been proposed many technologies in order to eliminate the problems caused by the reverse transfer phenomenon, another problems occurs with high cost for manufacturing the image forming apparatus, complexity of the apparatus and degradation of image quality. The problems due to the reverse transfer phenomenon can not be completely solved at the present stage.
Accordingly, it is a general object of the present invention is to provide a color image forming apparatus wherein the aforementioned problems are eliminated.
A more specific object of the present invention is to provide the color image forming apparatus wherein the problems due to the reverse transfer phenomenon can be resolved by simple constructions and with good manufacturing cost.
The above objects of the present invention are achieved by a color image forming apparatus in which a color image is formed by accumulating a visible image on an image transfer medium, comprising:
a plurality of forming means for forming the visible image on an image forming medium with a toner; and
a plurality of transferring means for transfer the visible image onto the image transfer medium while bringing the visible image into contact with the image transfer medium, wherein with respect to a toner amount which is transferred from each of the image forming medium onto the image transfer medium, difference between a toner deposition amount on the image transfer medium right after transferring the visible image from the image forming medium and a final toner deposition amount on the image transfer medium in the event of transfer of visible images is completed onto the image transfer medium is set such that the difference of the toner deposition amount of the visible image which is transferred by the transferring means disposed on an upstream side is larger than that on a downstream side in a direction where the visible image is successively formed.
The image transfer medium is a member for carrying the toner until a powder-like single color toner image is successively accumulated while transporting the plurality of transfer parts, so as to form a final full color toner image. For example, the intermediate transfer body corresponds to the above image transfer medium in the case of the image forming apparatus based on the single drum type. On the other hand, a sheet-like recording material such as a paper and a film or the like corresponds to the above image transfer medium in the case of the image forming apparatus based on the tandem type.
According to one aspect of the present invention, the toner image is formed with the largest deposition amount on the image transfer medium by means of the transferring means on the most upstream side. The toner deposition amount decreases in a downstream direction. After formation of the toner image on the image transfer medium, the toner deposition amount is changed according to the rotation number of the reverse transfer of respective toner image, so that an appropriate image density can be expressed.
According to other aspect of the present invention, since a decrease amount of the toner deposition of the toner image formed on the upstream side through the reverse transfer phenomenon is simply defined each time the toner image passes through one transferring means, the image forming apparatus can be realized with the simple construction.
The above objects of the present invention are also achieved by a color image forming apparatus provided with an unit for forming an image, in which a color image is formed by accumulating a visible image on an image transfer medium, the unit comprising:
a plurality of supplying means for supplying various types of toners;
at least one forming means for forming the visible image on an image forming medium with the toner; and
transferring means for transfer the visible image onto the image transfer medium while bringing the visible image into contact with the image transfer medium, wherein each of the toner which is stored in the respective supplying means includes an external additive and an addition amount, which is given by Sn, of the external additive contained in the respective supplying means is set such that there is an relationship of S1 less than S2 less than S3 less than S4 . . .  greater than Sn, when n represents the order from an upstream side.
According to further another aspect of the present invention, a function of increasing the toner deposition amount can be enhanced onto the image forming medium by increasing the addition amount of the external additive into the toner for use in the image forming means on the upstream side. Therefore, by the provision of the simple construction that a great volume of the external additive is added into the supplying means for the toner, the toner deposition amount can be adjusted in such a fashion that the toner deposition amount is increased in the toner image on the upstream side.
An advantage of the present invention is that a final toner deposition amount on the image transfer medium is maintained to a degree sufficient to express an appropriate image density for each of color toners, thereby inhibiting an influence caused by the reverse transfer of the toner, therefore providing the image forming apparatus capable of forming good images with image density being constant and proper color balance.
Another advantage of the present invention is that an increase in the toner deposition amount required for each toner can be attained by the simple construction in which a great volume of the external additive is added with the toner supplying means on the upstream side. Use of such a construction enables a conventional image forming apparatus to be manufactured with elimination of problems caused by the reverse transfer phenomenon. To be specific, the problems concerning the reverse transfer phenomenon can be resolved without increasing manufacturing cost.